Retroelements are genetic parasites such as viruses and transposons that multiply by incorporating their own reverse transcribed sequence into the genome of host cells. In eukaryotes, the wide-spread success of long terminal repeat (LTR)-containing retroelements has led to replication mechanisms that are conserved among diverse families of retrotransposons and retroviruses. The medical importance of retroviruses has intensified the need to understand the molecular details of the mechanisms responsible for the propagaion of LTR-retroelements. Although there is great evolutionary distance between the animals that are infected by retroviruses and organisms such as yeast that host LTR- retrotransposons, there is surprising conservation in the individual mechanisms these elements use to propagate. As a result, powerful techniques of yeast genetics can be applied to answer basic questions about LTR-retroelements. We have isolated mutagenized strains of S. pombe that were unable to support Tf1 transposition. One gene required for transposition, nup124, encodes a protein with 11 copies of FXFG, a repeat motif found in many nuclear pore factors. This year we have studied the localization of Nup124p and found that indeed it is concentrated within the nuclear pore complex. The key evidence for this are the results of immunofluorescence microscopy. More importantly, we found that strains with the nup124-1 mutation showed a significant defect in the nuclear import of Tf1 Gag. Despite the defect in Gag import, cells with nup124-1 exhibited normal growth rates and showed no defects in the nuclear import of other proteins. A complete deletion of nup124 shows these same properties. To understand the specific role of Nup124p in the nuclear import of Tf1, we tested individual regions of Gag for nuclear localization activity. Fifty amino acid domains were fused to GFP-LacZ and examined by fluorescence for cellular localization. One segment of 50 amino acids within Gag was found to cause nuclear localization of GFP-LacZ in a manor that required Nup124p. This NLS was found to be required in the context of the intact Tf1 transposon to cause nuclear import of Gag. We are currently testing these residues for interactions with other factors tthat may contribute to the import of Tf1. - retrotransposon, Tf1, nuclear import, reverse transcription, Schizosaccharomyces pombe